The present invention relates to a rotary joint for alternating passage of a lubricating medium and a non-lubricating medium from a stationary to a rotating machine part, having a first seal in the form of two flat, substantially circular ring-shaped sliding sealing surfaces arranged so as to slide one on the other, which sealing surfaces are substantially concentric relative to the axis of rotation of the rotating machine part and may be moved at least far enough from one another, in the axial direction, that they do not come into contact with each other during relative rotation of the rotating machine part.
Such rotary joints, which are also known as xe2x80x9cdry run-safe rotary jointsxe2x80x9d, are already known. If a lubricating medium is supplied, the two flat sliding sealing surfaces are brought into contact with each other, the lubricating medium forming a lubricating film between the sealing surfaces sliding one on the other, such that it is ensured in this way that excessive frictional heat does not arise even at high rotational speeds, which could otherwise lead very rapidly to destruction of the sliding sealing surfaces.
However, the sliding sealing surfaces have to be moved apart in the axial direction If the rotating machine part rotates at a certain rotational speed without the lubricating medium being fed thereto. In this case no lubricating film can form between the sliding sealing surfaces and the dry sliding sealing surfaces, which are generally made of a ceramic material, very rapidly become hot and may be destroyed thereby, even if the rotational speed of the rotating machine part is comparatively low and way below the maximum rotational speed at which the seal is designed to operate when lubricating medium is supplied.
Dry run safety is thus achieved in that the two flat sliding sealing surfaces are moved a short distance away from one another, the increasing sealing gap having in any case to be large enough to ensure that the sliding sealing surfaces do not come into contact during the period without lubricating medium even taking account of the tolerances which may be exhibited by all the components of the device.
A series of applications exists in which another medium, without lubricating properties, has also to be supplied during rotation of the rotating machine part. For example, a water/oil emulsion may be supplied to a machine tool as coolant and lubricant during a first operating phase through a hollow spindle, the one end of which is connected to the rotating part of the rotary joint or constitutes this rotating part, whilst, during another operating phase, compressed air may be supplied for example for blowing through a drilled hole or blowing clean the working area or a tool, while the tool continues to turn.
The deliberate enlargement of the sealing gap for the purpose of protecting the sliding sealing surfaces during supply of this non-lubricating medium has the disadvantage, however, that compressed air may escape through this sealing gap in very considerable quantities, which leads not only to a drop in pressure and thus to reduced efficiency of the compressed air supplied to an operating point but additionally to a considerable loss of energy.
Taking this prior art as basis, the object of the present invention is to provide a rotary joint of the above-mentioned type which, despite its so-called dry run safety, nonetheless exhibits only small leakage losses even during passage of a non-lubricating medium and even when the rotating machine part is operating at high rotational speeds.
This object is achieved in that, in addition to the first flat seal, an additional cylindrical seal is provided, the sealing surfaces of which are formed of cylindrical circumferential surfaces arranged concentrically relative to one another with a narrow sealing gap, the cylindrical seal being arranged in the direction of potential leakage flow between the flow passage of the rotary joint and the first seal.
The above-mentioned features ensure that the non-lubricating medium has to pass through the seal formed by cylindrical circumferential surfaces and the narrow sealing gap formed therebetween before it can pass at all through the enlarged axial gap of the flat seal, which narrow sealing gap may be kept substantially narrower and smaller than the axial gap between the flat sliding sealing surfaces in the event of feed of a non-lubricating medium.
The cylindrical sealing surfaces may therefore be made with substantially tighter, smaller tolerances, because very tight tolerances may be observed in the case of cylindrical surfaces having appropriately small diameters. In contrast, the axial distance between flat sliding sealing surfaces is influenced by all the components connected therewith, which may be relatively long, such that a substantially greater safety gap has to be established at the axial surfaces, in order to make the rotary joint dry run safe. In addition, any case in the preferred embodiment of the invention, the radius of the cylindrical circumferential surfaces is smaller than the Internal diameter of the flat, annular sliding sealing surfaces of the first seal. Because of the small radius of these cylindrical circumferential surfaces, the relative speeds between the mutually opposing sealing surfaces are also lower and only relatively little frictional heat therefore arises even in the case of slight contact.
In this way, it may be ensured that even a medium with very low viscosity, such as compressed air for example, may be conveyed through the rotary joint with a very low leakage rate, even if the rotating machine part rotates at high speeds of, for example, 20,000 revolutions/minute relative to the stationary machine part.
If the lubricating medium is supplied, said medium naturally also penetrates into the narrow radial sealing gap between the cylindrical sealing surfaces and thence also into the axial gap between the flat sliding sealing surfaces, wherein, in this case however, the sliding surfaces are pressed together and thus reduce the sealing gap virtually to the thickness of the lubricating film produced by the lubricating medium.
The outer cylindrical circumferential surface of the second seal is appropriately connected to the rotating machine part, while the inner cylindrical circumferential surface of the second seal is connected to the stationary machine part. It goes without saying that the reverse arrangement is in principle also possible.
In any case, however, it is appropriate for the cylindrical sealing surfaces to be so arranged that the inner cylindrical circumferential surface overlaps the area of he first flat seal in the axial direction. In other words, the inner cylindrical circumferential surface, located on a sleeve, is connected either to the stationary or to the rotating machine part and the seal with the flat sealing surfaces comprises this sleeve, wherein each of the flat sealing surfaces is respectively connected with the stationary machine part or the rotating machine part. The part of this sleeve or cylindrical wall projecting beyond the first flat seal in the axial direction forms the inner or outer sealing surface of the second seal.
It is also appropriate for the flat sealing surfaces to be resiliently prestressed against each other. This means that the flat sliding sealing surfaces normally contact each other with a well-defined contact pressure, such that the rotary joint is designed for the supply of a lubricating medium, while, for the supply of non-lubricating media, the sliding sealing surfaces have to be moved apart in the axial direction against the spring force. In many instances of application this is the shorter working cycle.
The respective sealing surfaces or the parts directly or indirectly carrying these sealing surfaces are preferably so designed that the flat sealing surfaces may be moved apart to an axial sealing gap width which is at least ten times the radial sealing gap width between the cylindrical circumferential surfaces of the second seal.
Conversely, it could also be said that the radial sealing gap width of the cylindrical circumferential surfaces amounts to at most a tenth of the necessary axial minimum gap width of the first seal, which has to be established for dry run safety, in any case for continuous running. Taking account of the smaller radius of the cylindrical circumferential surfaces compared with the flat sliding sealing surfaces and taking account of the axial flow path along the cylindrical sealing surfaces compared with the radial flow path between the flat sliding sealing surfaces, the above-mentioned construction ensures that the leakage rate in the construction according to the invention is reduced by at least the factor 100 compared with merely providing the flat seal with an enlarged sealing gap for dry run safety.
A preferred embodiment of the invention has rolling bearings incorporated into the rotary joint and a hollow shaft which is arranged rotatably in the rolling bearings and connected integrally with the outer cylindrical circumferential surface.
In this embodiment, particularly exact guidance of the cylindrical sealing surfaces may be achieved, such that the latter may be produced with particularly tight tolerances, it thus being possible to make the radial sealing gap particularly narrow.
As far as the flat sliding sealing surfaces are concerned, an embodiment of the invention is preferred in which these sliding sealing surfaces are arranged on sliding sealing rings, which may be connected in firm, leakproof manner to the stationary or rotating machine part respectively.
Since the sliding sealing surfaces are frequently made of a special material, for example ceramics, this arrangement the sliding sealing surfaces on separate rings, which may however be connected in firm and leakproof manner with the associated parts, is particularly expedient.
Furthermore, in a convenient embodiment of the invention either the inner or the outer cylindrical surface of the second seal is mounted so as to be resiliently elastic in the radial direction. In this way, slight tolerance variations and circularity errors exhibited by the mutually opposing sealing surfaces may be easily compensated, without resulting in excessive or significant friction and wear to the cylindrical surfaces.
For a gaseous or non-lubricating medium, it is expedient to provide a radial feed opening on the rotary joint according to the invention, while, for the lubricating medium, an axial feed opening is provided centrally at one end of the rotary joint, these positions being reversible without difficulty, however.
So that supply of the media may be effected independently and without mutual interference, the preferred embodiment of the invention in this case additionally provides for the feed opening for gas or another non-lubricating medium and also the feed opening for the lubricating medium to be secured in each case by non-return valves respectively against outflow of this or the other medium counter to the feed direction.
It is additionally expedient for the lubricating medium to be supplied axially and the non-lubricating medium to be supplied radially to a bushing, which is mounted not rotatably but axially displaceably in the stationary machine part and which carries the stationary machine part sealing surfaces of both the first and second seal. In this embodiment, pressure is exerted on the axially movable sleeve in the case of axial feed of the lubricating medium, whereby the non-return valve provided on said bushing opens and the bushing is pressed in the axial direction with its flat sliding sealing surface against the sliding sealing surface of the rotating machine part, such that, when the lubricating medium is supplied, the desired narrow sealing gap is automatically established. If supply of the lubricating medium is stopped, such that the pressure on the bushing in the axial direction eases and the non-lubricating medium is fed through the radial feed opening in the bushing instead, the pressure thereof not only opens the second non-return valve but also acts axially on the first non-return valve in the opposite direction from the lubricating medium, whereby the bushing is moved axially back and the two flat sliding sealing surfaces are thereby moved apart. In this way, the desired position of the sealing surfaces is established automatically by the alternating supply of lubricating and non-lubricating media.
If, however, the rotary joint is also to be used without feed of either of the two media, the bushing, carrying the sliding sealing surfaces, of the stationary machine part may also be prestressed by a spring against the compressive force of the lubricating medium, such that in the normal situation, in the absence of feed of the lubricating medium, the flat sliding sealing surfaces 4, 6 are moved apart and are not in contact with one another.